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Abstract

A gold nanorod-facilitated optical heating method for droplets in microfluidic chips is reported. Individual and stream nanoliter level droplets containing gold nanorods are heated by a low power 808-nm-wavelength laser. Owing to the high photothermal conversion efficiency of gold nanorods, a droplet temperature of 95 °C is achieved by employing a 13.6 mW laser with good reproducibility. The heating and cooling times are 200 and 800 ms, respectively, which are attributed to the fast thermal-transfer rates of the droplets. By controlling the irradiation laser power, the temperature cycles for polymerase chain reaction are also demonstrated.

(a) Fluorescence spectra of rhodamine B in the droplet obtained at increasing laser powers. The insets are optical microscope images of a droplet containing GNRs taken without (top) and with (bottom) a 13.6 mW 808-nm-wavelength laser heating. The scale bars: 100 μm. (b) Temperature of the droplet as a function of laser power.

(a) Reversible response of the droplet temperature when the laser was switched on (11 mW) and off alternatively. (b) Typical time-dependent temperature of the droplet reveals the heating and cooling times of about 200 and 800 ms, respectively.